In cellular telephone or other wireless voice or data systems, a served area is divided into cells, each of which may be further divided into sectors. Each cell may be served by a single base station, which is further connected to a message switching center (MSC), a subscriber management system (SMS), or a data router via a base station controller (BSC). A plurality of wireless communication devices/terminals are connected to the MSC, the SMS, or the router by establishing radio links with one or more nearby base stations.
In earlier cellular telephone technology, such as time division multiple access (TDMA), as a wireless mobile terminal travels from one cell to another, the radio link between the terminal and the base station serving the first cell had to be broken and then replaced by a radio link between the terminal and the base station serving the second cell. In contrast, in a code division multiple access (CDMA) cellular telephone system, because the same frequency band is used for all cells and sectors, the first link need not be broken before connecting with the second link.
In a Code Division Multiple Access (CDMA) system, signals from a plurality of mobile terminals are mixed and overlapped in time and frequency domains. Since certain correlations exist between signals of individual mobile terminals, signals of different mobile terminals at the receiving end will interfere with each other, and such interference is commonly called Multi-Access Interference (MAI). Although the MAI generated by a mobile terminal is very small, as the number of mobile terminals or signal power increases, MAI becomes one of major interferences in a CDMA system.
A TD-SCDMA system inhibits the MAI through a technique called multi-mobile terminal joint detection in order to increase anti-interference ability of the system. Multi-mobile terminal joint detection no longer treats signals from other terminals blindly as interference signals. Instead, it fully utilizes midamble information included in the received signals, such as the channel impulse response of each mobile terminal, and concurrently extracts signals of all the mobile terminals from the received signals. With the multi-mobile terminal joint detection, anti-interference ability of TD-SCDMA systems is largely enhanced, as well as system capacity and coverage.
In the current multi-mobile terminal joint detecting technique, the TD-SCDMA system firstly determines impulse responses of mobile terminals based on a predetermined training sequence known as a midamble sent by the mobile terminals in normal timeslots, and determines respective channel estimation windows of the mobile terminal based on cyclic offsets of the midamble. Upon determining the location of respective channel estimation windows of the mobile terminals, a base station is able to extract channel impulse responses of the mobile terminals from the channel impulse responses of the mobile terminals under its coverage. Thereafter, the multi-mobile terminal joint detection can be performed based on the channel impulse responses of the mobile terminals.
One prerequisite for performing multi-mobile terminal joint detection is that the channel impulse responses of the mobile terminals must first be obtained. In the TD-SCDMA system, each timeslot of each subframe includes two data fields and one training sequence field. The midamble distributed by the system is sent in the training sequence field of an uplink timeslot, so as to allow the base station to perform channel estimation and synchronization control. The TD-SCDMA system assigns a basic midamble for each cell, where different cells use different basic midamble which have certain orthogonal characteristics. Midambles used by different mobile terminals in the same cell are obtained by cyclic shifting or offsetting the basic midamble of the cell, and the cyclic offsets of different mobile terminals are different. Thus, the base station can simultaneously estimate the channel impulse responses of all mobile terminals in the current cell by distinguishing midambles sent by mobile terminals in the current cell and those of neighboring cells by certain operations through the use of basic midamble of the current cell. Since the midambles used by different mobile terminals in the same cell have different cyclic offsets, thus the channel impulse responses of different mobile terminals in the same cell will have different time delays. In order to receive and detect signals from various mobile terminals, their channel impulse responses are obtained and processed through various corresponding channel estimation windows. That is, the channel impulse responses of different mobile terminals in the same cell should be located at different channel estimation windows. Respective channel estimation windows of mobile terminals can be determined by the cyclic offset of midamble used by the respective mobile terminal. After the channel estimation window of each mobile terminal is determined, the base station can respectively segment the channel impulse response of each individual mobile terminal from the channel impulse responses of all mobile terminals in the current cell. The multi-mobile terminal joint detection can then be performed based on each mobile terminal's channel impulse response.
However, it can be seen from the above process of determining channel impulse responses, a base station currently can only determine the uplink channel impulse responses of all mobile terminals in its cell, and therefore, can only perform multi-mobile terminal joint detection on uplink signals of all mobile terminals in the current cell. In a particular cell, since the mobile terminals from the neighboring cells are synchronized with the base stations in the corresponding neighboring cells, they, and other received uplink signals of mobile terminals from the neighboring cells are entirely treated as unknown MAIs.
What is needed is an improved method and system for effectively reducing multi-cell interferences by identifying impulse responses of a mobile terminal from a neighboring cell for effectively reducing MAI and achieving better communication quality.